1. Field of the Invention
Embodiments of the invention generally relate to electronic data storage and retrieval systems having magnetic heads capable of reading recorded information stored on magnetic media.
2. Description of the Related Art
In an electronic data storage and retrieval system, a magnetic head typically includes a reader portion having a magnetoresistive (MR) sensor for retrieving magnetically-encoded information stored on a magnetic recording medium or disk. The MR sensor operates based on a change of electrical resistivity of certain materials of the MR sensor in the presence of a magnetic field. During a read operation, a bias current is passed through the MR sensor. Magnetic flux emanating from a surface of the recording medium causes rotation of a magnetization vector of a sensing layer of the MR sensor, which in turn causes the change in electrical resistivity of the MR sensor. Since a voltage across the MR sensor is equal to the bias current that is supplied times the resistivity, the change in electrical resistivity of the MR sensor can be detected by measuring a voltage across the MR sensor to provide voltage information that external circuitry can then convert and manipulate as necessary.
To efficiently read data from a data track of the recording medium, the MR sensor of the magnetic head must be shielded from extraneous magnetic fields, such as those generated by a write head or adjacent data tracks. Accordingly, the MR sensor is sandwiched between a pair of magnetic shields within the read head portion. During the read operation, first and second read shields ensure that the MR sensor reads only the information stored directly beneath it on a specific track of the recording medium by. absorbing any stray magnetic fields.
Each of the shields typically includes one or more layers of ferromagnetic materials such as a nickel iron alloy. The ferromagnetic materials within the shields possess high coefficients of thermal expansion relative to most other materials in the magnetic head. Further, the amount of thermal expansion of the shields occurs proportionally to the volume of the shield, especially since a bulk of the volume is typically the ferromagnetic materials. The MR sensor is relatively small in volume compared to the shields. Consequently, an expansion differential occurs between the MR sensor and the shields as the temperature of the magnetic head increases. Specifically, the shields experience relatively more expansion than the MR sensor with increasing temperature. The difference in expansion results in the MR sensor becoming recessed along an air bearing surface of the magnetic head relative to the protruding shields at the air bearing surface. This recessing of the MR sensor due to thermal expansion of the shields adversely effects separation of the MR sensor from the recording medium resulting in increased read errors.
A contributing factor to the problem of shield expansion is that the shields conforming to conventional structures utilize thickness of the layers of the ferromagnetic materials to maintain a magnetic stiffness desired. Ensuring shield magnetic stability avoids non-linear response of the MR sensor and increases in write induced instability/popcorn noise. The magnetic stiffness of the shields according to prior configurations for the shields decreases with a reduction in thickness of the shields. Accordingly, this thickness of the ferromagnetic materials adds to the volume of the shields, thereby contributing to relatively large thermal expansion of the shields.
Therefore, there exists a need for an improved shield structure that eliminates or reduces temperature dependent protrusion of the shield into a planar air bearing surface common with an end surface of an MR sensor.